A golf club is generally formed by affixing a club head to a first end of a flexible shaft, and affixing a grip member to a second end of the shaft. Convention and the USGA Rules of Golf have established certain terminology to describe different portions and angular relationships of a club head. For example, a wood-type club head includes a face or striking face, a crown, a sole, a heel, a toe, a back, and a hosel. These club head portions are most easily described when the club head is positioned in a reference position relative to a ground plane. In the reference position, the lie angle of the club (i.e., the angle formed between the shaft and the ground plane) and the loft angle of the club (i.e., the angle formed between the face and the ground plane) are oriented as specified by the manufacturer.
The sole of the club head is generally disposed on an opposite side of the club head from the crown, and is further disposed on an opposite side of the club head from the shaft. When in the reference position, the sole of the club head is intended to contact the ground plane. For the portion of the club that is to the rear of the face, the crown may be separated from the sole at the point on the club head where the surface tangent of the club head is normal to the ground plane.
The hosel is the portion of the club head that is intended to couple the club head with the shaft. The hosel includes an internal bore that is configured to receive the shaft or a suitable shaft adapter. In a configuration where the shaft is directly inserted into the hosel, the hosel bore may have a center hosel-axis that is substantially coincident with a center longitudinal-axis of the shaft. For club head embodiments including a shaft adapter, the shaft may be received in a suitable shaft adapter bore that has a center adapter-axis, which may be substantially coincident with the shaft axis. The shaft adapter-axis may be offset angularly and/or linearly from the hosel-axis to permit adjustment of club parameters via rotation of the shaft adapter with respect to club head, as is known by persons skilled in the art.
The heel may be defined as the portion of the club head that is proximate to and including the hosel. Conversely, the toe may be the area of the golf club that is the farthest from the shaft. Finally, the back of the club head may be the portion of the club head that is generally opposite the face.
Two key parameters that affect the performance and forgiveness of a club include the magnitude and location of the club head's center of gravity (COG) and the various moments of inertia (MOI) about the COG. The club's moments of inertia relate to the club's resistance to rotation (particularly during an off-center hit). These are often perceived as the club's measure of “forgiveness.” In typical driver designs, high moments of inertia are desired to reduce the club's tendency to push or fade a ball. Achieving a high moment of inertia generally involves placing mass as close to the perimeter of the club as possible (to maximize the moment of inertia about the center of gravity), and as close to the toe as possible (to maximize a separate moment of inertia about the shaft).
While the various moments of inertia affect the forgiveness of a club head, the location of the center of gravity can also affect the trajectory of a shot for a given face loft angle. For example, a center of gravity that is positioned as far rearward (i.e., away from the face) and as low (i.e., close to the sole) as possible typically results in a ball flight that has a higher trajectory than a club head with a center of gravity placed more forward and/or higher.
While a high moment of inertia is obtained by increasing the perimeter weighting of the club head, an increase in the total mass/swing weight of the club head (i.e., the magnitude of the center of gravity) has a strong, negative effect on club head speed and hitting distance. Said another way, to maximize club head speed (and hitting distance), a lower total mass is desired; however a lower total mass generally reduces the club head's moment of inertia (and forgiveness).
The desire for a faster swing speed (i.e., lower mass) and greater forgiveness (i.e., larger MOI or specifically placed COG) presents a difficult optimization problem. These competing constraints explain why most drivers/woods are formed from hollow, thin-walled bodies, with nearly all of the mass being positioned as far from the COG as possible (i.e., to maximize the various MOI's). Additionally, removable/interchangeable weights have been used to alter other dynamic, swing parameters and/or to move the COG. Therefore, the total of all club head mass is the sum of the total amount of structural mass and the total amount of discretionary mass. Typical driver designs generally have a total club head mass of from about 195 g to about 215 g.
Structural mass generally refers to the mass of the materials that are required to provide the club head with the structural resilience needed to withstand repeated impacts. Structural mass is highly design-dependant, and provides a designer with a relatively low amount of control over specific mass distribution.
Discretionary mass is any additional mass (beyond the minimum structural requirements) that may be added to the club head design for the sole purpose of customizing the performance and/or forgiveness of the club. In an ideal club design, for a constant total swing weight, the amount of structural mass would be minimized (without sacrificing resiliency) to provide a designer with additional discretionary mass to customize club performance.
While this provided background description attempts to clearly explain certain club-related terminology, it is meant to be illustrative and not limiting. Custom within the industry, rules set by golf organizations such as the United States Golf Association (USGA) or the R&A, and naming convention may augment this description of terminology without departing from the scope of the present application.